Planetary imagers, especially orbital earth imagers, are useful for remote sensing of atmospheric composition, geologic morphology and chemistry, crop assessment, weather prediction, and monitoring the activities of man. Monochromatic and multispectral satellite based imagers can quantify properties of the above earth characteristics, provided their solid state detector arrays are properly calibrated it relation to radiometric responsivities.
One prior art method is schematically illustrated in FIG. 2. This method utilizes a radiometric calibration assembly 9 having a an entrance port 14, a flat fold mirror 16, a lens 18, and a perforated plate 30. The perforated plate 30 is shown in detail in FIG. 3. A plurality of small apertures 32, 34, 36, 38 are formed in the otherwise opaque plate. Each aperture is of different size, but each is necessarily smaller than the image of the sun 12 formed by lens 18 (see FIG. 2). In execution of the radiometric calibration, the sun's image is caused to move to, and park upon each aperture in turn. For example, the fold mirror 16 can be steered to place a solar image 40 (see FIG. 3) upon smallest aperture 32, resulting in a discrete lowest calibration flux level being delivered to an imaging sensor array 10. The array responsivity to this flux level is measured and then fold mirror 16 can be steered to next larger aperture 34, resulting in a discrete higher calibration flux level. The array responsivity to a sequence of discrete and increasing flux levels, representative of the range of anticipated imaging flux levels is thus achieved. Detractors from this approach include time wasted in moving the solar image from aperture to aperture, and the disjoint nature of the motion from park to move to park in laborious repetition. Only a small portion of the total calibration time available is actually used in collecting flux measurements, thus detracting from the potential accuracy gainable by statistical averaging if more flux collection time were available. In addition, only one aperture can be geometrically centered, resulting in some differential nonuniformity in the flux level at the imaging array 10, as different apertures are accessed. Lastly, the discrete small number of steps characteristic of the small finite number of apertures necessitates interpolation of the inferred responsivity of the array between calibration steps, degrading accuracy in these regions.